Mobile wireless communications device with reduced harmonics resulting from metal shield coupling

ABSTRACT

A mobile wireless communications device includes a housing, antenna, and circuit board carried by the housing and having radio frequency (RF) circuitry operative with the antenna for receiving and transmitting RF signals through the antenna. A power amplifier is connected within a transmission line for amplifying RF signals to be transmitted over the transmission line to the antenna. An antenna switch is connected to the antenna and RF circuitry. An RF shield surrounds the power amplifier and antenna switch and isolates the power amplifier and antenna switch from the antenna and RF circuitry. A low pass filter is connected to the power amplifier and antenna switch for reducing any RF coupling of voltage standing waves of upper harmonic frequencies from the power amplifier into the antenna switch through the RF shield while maintaining transmission of signals through the transmission line at a desired fundamental frequency.

RELATED APPLICATIONS

This application is a continuation of Ser. No. 13/325,099 filed Dec. 14,2011, which, in turn, is a continuation of Ser. No. 12/117,036 filed May8, 2008 now U.S. Pat. No. 8,099,064 issued Jan. 17, 2012, all of whichare hereby incorporated herein in their entireties by reference.

FIELD OF THE INVENTION

The present invention relates to the field of communications devices,and more particularly, to mobile wireless communications devices andrelated methods.

BACKGROUND OF THE INVENTION

Cellular communication systems continue to grow in popularity and havebecome an integral part of both personal and business communications.Cellular telephones allow users to place and receive phone calls mostanywhere they travel. Moreover, as cellular telephone technology isincreased, so too has the functionality of cellular devices. Forexample, many cellular devices now incorporate Personal DigitalAssistant (PDA) features such as calendars, address books, task lists,calculators, memo and writing programs, etc. These multi-functiondevices usually allow users to wirelessly send and receive electronicmail (email) messages and access the internet via a cellular networkand/or a wireless local area network (WLAN), for example.

As the functionality of cellular communications devices continues toincrease, so too does demand for smaller devices that are easier andmore convenient for users to carry. As any circuit boards and electroniccomponents thereon are reduced in size and placed closer together,including antenna and other RF components, including power amplifiersand antenna switches, various electronic components can pick upconductive energy and create interference within the system. Forexample, some components could pick up conducted energy directly from apower amplifier or from the radiated energy emitted by an antenna. Thisunwanted reception of conducted/near field radiated energy from poweramplifiers and antennae is particularly problematic in a packet bursttransmission as part of a Global System for Mobile communications (GSM)system, including the 450 MHz, 900 MHz, 1800 MHz and 1900 MHz frequencybands.

Some mobile wireless communications devices have an RF metal shield alsotermed a “can” that forms a compartment on a circuit board and receivesRF circuitry therein, for example, the power amplifier and antennaswitch, typically a diplexer antenna switch also termed atransmit/receiver antenna switch. One or more RF shields as metallic“cans” can form radio frequency isolation compartments that may includea transceiver chip set in one “can” and the power amplifier and antennaswitch in another “can” to aid RF filtering between the RF poweramplifier and the antenna switch. Some signal coupling through the RFshield as created from voltage standing waves of single or multipleharmonics, thus bypassing various components such as filters that areformed to prevent such coupling.

BRIEF DESCRIPTION OF THE DRAWINGS

Other objects, features and advantages will become apparent from thedetailed description which follows, when considered in light of theaccompanying drawings in which;

FIG. 1 is a schematic block diagram of an example of a mobile wirelesscommunications device configured as a handheld device and illustratingbasic internal components thereof.

FIG. 2 is a front elevation view of the mobile wireless communicationsdevice of FIG. 1.

FIG. 3 is a schematic block diagram showing basic functional circuitcomponents that can be used in the mobile wireless communications deviceof FIGS. 1-2.

FIG. 4 is front elevational view of the mobile wireless communicationsdevice in accordance with one embodiment having the front cover removedto illustrate an example of RF circuitry, power amplifier, surfacemounted microphone and noise isolation components associated thereof.

FIG. 5 is a fragmentary, side elevation view showing the RF shield onthe printed circuit board and showing in schematic representation the RFcoupling between the input and output of a typical LC low pass filtervia the RF shield because of standing waves at harmonic frequencies.

FIG. 6 is a schematic circuit diagram of the circuit shown in FIG. 5 andshowing a typical LC low pass harmonic filter and the input and outputstanding waves resulting from a mismatch at harmonic frequencies.

FIG. 7 is a graph showing the corresponding return loss and plot of anLC low pass harmonic filter over fundamental and harmonic frequenciesfor the circuit of FIGS. 5 and 6.

FIG. 8 is a graph showing a corresponding insertion loss plot of an LClow pass harmonic filter over fundamental and harmonic frequencies forthe circuit of FIGS. 5 and 6.

FIG. 9 is a schematic circuit diagram showing an RLC low pass harmonicfilter that results in an improvement of input and output standing wavesat harmonic frequencies.

FIG. 10 is a graph showing the corresponding return loss plot of the RLClow pass harmonic filter over fundamental and harmonic frequencies basedupon the circuit shown in FIG. 9.

FIG. 11 is a graph showing the corresponding insertion loss plot of theRLC low pass harmonic filter over fundamental and harmonic frequencies.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present description is made with reference to the accompanyingdrawings, in which preferred embodiments are shown. However, manydifferent embodiments may be used, and thus the description should notbe construed as limited to the embodiments set forth herein. Rather,these embodiments are provided so that this disclosure will be thoroughand complete. Like numbers refer to like elements throughout.

Typically, a low pass filter exists between the power amplifier andantenna switch. An RF shield such as a metallic “can” encases the poweramplifier and antenna switch and includes therein a three-pole LC filterin the middle of the power amplifier and antenna switch transmissionline. In the tight physical space under the RF “can”, the transmissionline can be mismatched at harmonic frequencies and create significantstanding waves. The RF shield couples the RF harmonics from a poweramplifier output directly to the antenna switch input, causing problemssuch that signals bypass any low pass filter. This RF coupling becauseof the RF shield is created by the high RF voltage standing wave ratio(VSWR).

In accordance with one non-limiting aspect, a mobile wirelesscommunications device includes a housing and antenna supported by thehousing. A circuit board is carried by the housing and has radiofrequency (RF) circuitry operative with the antenna for receiving andtransmitting RF signals through the antenna. A power amplifier isconnected within a transmission line for amplifying RF signals to betransmitted over the transmission line to the antenna. An antenna switchis carried by a circuit board and connected to the antenna for switchingbetween receiving and transmitting RF signals to the antenna. An RFshield surrounds the power amplifier and antenna switch for isolatingthe power amplifier and antenna switch from the antenna and RFcircuitry. In accordance with a non-limiting example, a low pass filteris formed within the transmission line and has an input connected to thepower amplifier and an output connected to the antenna switch forreducing any RF coupling of voltage standing waves of upper harmonicfrequencies from the power amplifier into the antenna switch between theinput and output of the low pass filter through the RF shield whilemaintaining transmission of signals through the transmission line at adesired fundamental frequency.

In one aspect, a low pass filter can be formed as an RLC filter andformed using a capacitor and first resistor connected in a series and aninductor and second resistor connected in parallel. The low pass filtercould also include a grounded resistor in series with a first capacitorand a grounded second capacitor in parallel with the first capacitor.The values for each capacitor, resistor inductor can be chosen such thatat a fundamental frequency, the impedance of the capacitance is higherthan the first resistor, and the impedance of the inductor is lower thanthe second resistor. At higher harmonic frequencies, the impedance ofthe capacitor is reduced and the first resistor becomes a dominantelement for reducing any RF coupling of voltage standing waves of upperharmonic frequencies from the power amplifier into the antenna switchbetween the input and output of the low pass filter through the RFshield while maintaining transmission of signals through thetransmission line at a desired fundamental frequency.

In another aspect, the RF shield is formed as a metal housing secured toa circuit board and surrounding the power amplifier and antenna switch,which can be positioned within an isolation compartment on the circuitboard. The RF shield surrounds the power amplifier and antenna switchand can be positioned over the isolation compartment and secured to thecircuit board. The RF circuitry can be operative for generating GlobalSystem for Mobile (GSM) packet bursts. The RF circuitry can be formed asa transceiver chip set.

A method aspect is also set forth.

A brief description will now proceed relative to FIGS. 1-3, whichdisclose an example of a mobile wireless communications device, forexample, a handheld portable cellular radio, which can incorporate thenon-limiting examples of the various circuits as later described. FIGS.1-3 are representative non-limiting examples of the many different typesof functional circuit components and their interconnection, andoperative for use with the circuits as described.

Referring initially to FIGS. 1 and 2, an example of a mobile wirelesscommunications device 20, such as a handheld portable cellular radiothat can incorporate the low pass filter as described below is setforth. This device 20 illustratively includes a housing 21 having anupper portion 46 and a lower portion 47, and at least one dielectricsubstrate (i.e., circuit board) 67, such as a conventional printedcircuit board (PCB) substrate, for example, carried by the housing. Anumber of different circuit boards can be used for supporting differentcomponents. A housing cover (not shown in detail) would typically coverthe front portion of the housing. The term circuit board 67 as usedhereinafter can refer to any dielectric substrate, PCB, ceramicsubstrate or other circuit carrying structure for carrying signalcircuits and electronic components within the mobile wirelesscommunications device 20. The illustrated housing 21 is a statichousing, for example, but it should be understood that a flip or slidinghousing can be used as is typical in many cellular and similartelephones. These and other housing configurations may also be used.

Circuitry 48 is carried by the circuit board 67, such as amicroprocessor, memory, one or more wireless transceivers (e.g.,cellular, WLAN, etc.), which includes RF circuitry, including audio andpower circuitry, including any keyboard circuitry. This circuitry couldalso generally be termed RF circuitry. It should be understood thatkeyboard circuitry could be on a separate keyboard, etc., as will beappreciated by those skilled in the art. The different components asdescribed can also be distributed on one circuit board or among aplurality of circuit boards. A battery (not shown) is also preferablycarried by the housing 21 for supplying power to the circuitry 48. Theterm RF circuitry could encompass the interoperable RF transceivercircuitry, including receive and transmit circuits and power circuitryand audio circuitry.

In one aspect, an audio output transducer 49 (e.g., a speaker) iscarried by an upper portion 46 of the housing 21 and connected to thecircuitry 48. One or more user input interface devices, such as a keypad(keyboard) 23 (FIG. 2), is also preferably carried by the housing 21 andconnected to the RF circuitry 48. The term keypad as used herein alsorefers to the term keyboard, indicating the user input devices havinglettered and/or numbered keys commonly known and other embodiments,including multi-top or predictive entry modes. Other examples of userinput interface devices include a scroll wheel 37 and a back button 36.Of course, it will be appreciated that other user input interfacedevices (e.g., a stylus or touch screen interface) may be used in otherembodiments.

An antenna 45 is preferably supported within the housing and in oneaspect at a lower portion 47 in the housing. The antenna can be formedas a pattern of conductive traces that make an antenna circuit, whichphysically forms the antenna. It is operatively connected to thecircuitry 48 on the main circuit board 67. In one non-limiting example,the antenna could be formed on an antenna circuit board section thatextends from the circuit board at the lower portion of the housing. Byplacing the antenna 45 adjacent the lower portion 47 of the housing 21,the distance is advantageously increased between the antenna and theuser's head when the phone is in use to aid in complying with applicableSAR requirements. Also, a separate keyboard circuit board could be used.

More particularly, a user will typically hold the upper portion of thehousing 21 very close to their head so that the audio output transducer49 is directly next to the ear. Yet, the lower portion 47 of the housing21 where an audio input transducer (i.e., microphone) is located neednot be placed directly next to a user's mouth, and can be held away fromthe user's mouth. That is, holding the audio input transducer close tothe user's mouth may not only be uncomfortable for the user, but it mayalso distort the user's voice in some circumstances. In addition, theplacement of the antenna 45 adjacent the lower portion 47 of the housing21 also advantageously spaces the antenna farther away from the user'sbrain.

In some designs, the antenna 45 is placed adjacent the lower portion 47of the housing 21 to allow for less impact on antenna performance due toblockage by a user's hand. Users typically hold cellular phones towardsthe middle to upper portion of the phone housing, and are therefore morelikely to put their hands over such an antenna than they are an antennamounted adjacent the lower portion 47 of the housing 21. Accordingly,more reliable performance may be achieved from placing the antenna 45adjacent the lower portion 47 of the housing 21.

Another benefit of this type of configuration is that it provides moreroom for one or more auxiliary input/output (I/O) devices 50 to becarried at the upper portion 46 of the housing. Furthermore, byseparating the antenna 45 from the auxiliary I/O device(s) 50, this mayallow for reduced interference therebetween.

Some examples of auxiliary I/O devices 50 include a WLAN (e.g.,Bluetooth, IEEE 802.11) antenna for providing WLAN communicationcapabilities, and/or a satellite positioning system (e.g., GPS, Galileo,etc.) antenna for providing position location capabilities, as will beappreciated by those skilled in the art. Other examples of auxiliary I/Odevices 50 include a second audio output transducer (e.g., a speaker forspeaker phone operation), and a camera lens for providing digital cameracapabilities, an electrical device connector (e.g., USB, headphone,secure digital (SD) or memory card, etc.).

It should be noted that the term “input/output” as used herein for theauxiliary I/O device(s) 50 means that such devices may have input and/oroutput capabilities, and they need not provide both in all embodiments.That is, devices such as camera lenses may only receive an opticalinput, for example, while a headphone jack may only provide an audiooutput.

The device 20 further illustratively includes a display 22, for example,a liquid crystal display (LCD) carried by the housing 21 and connectedto the circuitry 48. A back button 36 and scroll wheel 37 can also beconnected to the circuitry 48 for allowing a user to navigate menus,text, etc., as will be appreciated by those skilled in the art. Thescroll wheel 37 may also be referred to as a “thumb wheel” or a “trackwheel” in some instances. The keypad 23 illustratively includes aplurality of multi-symbol keys 24 each having indicia of a plurality ofrespective symbols thereon. The keypad 23 also illustratively includesan alternate function key 25, a next key 26, a space key 27, a shift key28, a return (or enter) key 29, and a backspace/delete key 30.

The next key 26 is also used to enter a “*” symbol upon first pressingor actuating the alternate function key 25. Similarly, the space key 27,shift key 28 and backspace key 30 are used to enter a “0” and “#”,respectively, upon first actuating the alternate function key 25. Thekeypad 23 further illustratively includes a send key 31, an end key 32,and a convenience (i.e., menu) key 39 for use in placing cellulartelephone calls, as will be appreciated by those skilled in the art.

Moreover, the symbols on each key 24 are arranged in top and bottomrows. The symbols in the bottom rows are entered when a user presses akey 24 without first pressing the alternate function key 25, while thetop row symbols are entered by first pressing the alternate functionkey. As seen in FIG. 2, the multi-symbol keys 24 are arranged in thefirst three rows on the keypad 23 below the send and end keys 31, 32.Furthermore, the letter symbols on each of the keys 24 are arranged todefine a QWERTY layout. The letters on the keypad 23 are presented in athree-row format, with the letters of each row being in the same orderand relative position as in a standard QWERTY keypad.

Each row of keys (including the fourth row of function keys 25-29) isarranged in five columns in this non-limiting example. The multi-symbolkeys 24 in the second, third, and fourth columns of the first, second,and third rows have numeric indicia thereon (i.e., 1 through 9)accessible by first actuating the alternate function key 25. Coupledwith the next, space, and shift keys 26, 27, 28, which respectivelyenter a “*”, “0”, and “#” upon first actuating the alternate functionkey 25, as noted above, this set of keys defines a standard telephonekeypad layout, as would be found on a traditional touch-tone telephone,as will be appreciated by those skilled in the art.

Accordingly, the mobile wireless communications device 20 as describedmay advantageously be used not only as a traditional cellular phone, butit may also be conveniently used for sending and/or receiving data overa cellular or other network, such as Internet and email data, forexample. Of course, other keypad configurations may also be used inother embodiments. Multi-tap or predictive entry modes may be used fortyping e-mails, etc. as will be appreciated by those skilled in the art.

The antenna 45 is preferably formed as a multi-frequency band antenna,which provides enhanced transmission and reception characteristics overmultiple operating frequencies. More particularly, the antenna 45 isdesigned to provide high gain, desired impedance matching, and meetapplicable SAR requirements over a relatively wide bandwidth andmultiple cellular frequency bands. By way of example, in onenon-limiting example, the antenna 45 preferably operates over fivebands, namely a 850 MHz Global System for Mobile Communications (GSM)band, a 900 MHz GSM band, a DCS band, a PCS band, and a WCDMA band(i.e., up to about 2100 MHz), although it may be used for otherbands/frequencies as well. To conserve space, the antenna 45 mayadvantageously be implemented in three dimensions although it may beimplemented in two-dimensional or planar embodiments as well.

The mobile wireless communications device shown in FIGS. 1 and 2 canincorporate e-mail and messaging accounts and provide differentfunctions such as composing e-mail, PIN messages, and SMS messages. Thedevice can manage messages through an appropriate menu that can beretrieved by choosing a messages icon. An address book function couldadd contacts, allow management of an address book, set address bookoptions and manage SIM card phone books. A phone menu could allow forthe making and answering of phone calls using different phone features,managing phone call logs, setting phone options, and viewing phoneinformation. A browser application could permit the browsing of webpages, configuring a browser, adding bookmarks, and changing browseroptions. Other applications could include a task, memo pad, calculator,alarm and games, as well as handheld options with various references.

A calendar icon can be chosen for entering a calendar program that canbe used for establishing and managing events such as meetings orappointments. The calendar program could be any type of messaging orappointment/meeting program that allows an organizer to establish anevent, for example, an appointment or meeting.

A non-limiting example of various functional components that can be usedin the exemplary mobile wireless communications device 20 of FIGS. 1 and2 is further described in the example below with reference to FIG. 3.The device 20 illustratively includes a housing 120, a keypad 140 and anoutput device 160. The output device 160 shown is preferably a display,which is preferably a full graphic LCD. Other types of output devicesmay alternatively be used. A processing device 180 is contained withinthe housing 120 and is coupled between the keypad 140 and the display160. The processing device 180 controls the operation of the display160, as well as the overall operation of the mobile device 20, inresponse to actuation of keys on the keypad 140 by the user.

The housing 120 may be elongated vertically, or may take on other sizesand shapes (including clamshell housing structures). The keypad mayinclude a mode selection key, or other hardware or software forswitching between text entry and telephony entry.

In addition to the processing device 180, other parts of the mobiledevice 20 are shown schematically in FIG. 3. These include acommunications subsystem 101; a short-range communications subsystem102; the keypad 140 and the display 160, along with other input/outputdevices 106, 108, 110 and 112; as well as memory devices 116, 118 andvarious other device subsystems 121. The mobile device 20 is preferablya two-way RF communications device having voice and data communicationscapabilities. In addition, the mobile device 20 preferably has thecapability to communicate with other computer systems via the Internet.

Operating system software executed by the processing device 180 ispreferably stored in a persistent store, such as the flash memory 116,but may be stored in other types of memory devices, such as a read onlymemory (ROM) or similar storage element. In addition, system software,specific device applications, or parts thereof, may be temporarilyloaded into a volatile store, such as the random access memory (RAM)118. Communications signals received by the mobile device may also bestored in the RAM 118.

The processing device 180, in addition to its operating systemfunctions, enables execution of software applications 130A-130N on thedevice 20. A predetermined set of applications that control basic deviceoperations, such as data and voice communications 130A and 130B, may beinstalled on the device 20 during manufacture. In addition, a personalinformation manager (PIM) application may be installed duringmanufacture. The PIM is preferably capable of organizing and managingdata items, such as e-mail, calendar events, voice mails, appointments,and task items. The PIM application is also preferably capable ofsending and receiving data items via a wireless network 141. Preferably,the PIM data items are seamlessly integrated, synchronized and updatedvia the wireless network 141 with the device user's corresponding dataitems stored or associated with a host computer system.

Communication functions, including data and voice communications, areperformed through the communications subsystem 101, and possibly throughthe short-range communications subsystem. The communications subsystem101 includes a receiver 150, a transmitter 152, and one or more antennae154 and 156. In addition, the communications subsystem 101 also includesa processing module, such as a digital signal processor (DSP) 156, andlocal oscillators (LOs) 161. The specific design and implementation ofthe communications subsystem 101 is dependent upon the communicationsnetwork in which the mobile device 20 is intended to operate. Forexample, the mobile device 20 may include a communications subsystem 101designed to operate with the Mobitex™, Data TAC™ or General Packet RadioService (GPRS) mobile data communications networks, and also designed tooperate with any of a variety of voice communications networks, such asAMPS, TDMA, CDMA, PCS, GSM, etc. Other types of data and voice networks,both separate and integrated, may also be utilized with the mobiledevice 20.

Network access requirements vary depending upon the type ofcommunication system. For example, in the Mobitex and DataTAC networks,mobile devices are registered on the network using a unique personalidentification number or PIN associated with each device. In GPRSnetworks, however, network access is associated with a subscriber oruser of a device. A GPRS device therefore requires a subscriber identitymodule, commonly referred to as a SIM card, in order to operate on aCPRS network.

When required network registration or activation procedures have beencompleted, the mobile device 20 may send and receive communicationssignals over the communication network 141. Signals received from thecommunications network 141 by the antenna 154 are routed to the receiver150, which provides for signal amplification, frequency down conversion,filtering, channel selection, etc., and may also provide analog todigital conversion. Analog-to-digital conversion of the received signalallows the DSP 158 to perform more complex communications functions,such as demodulation and decoding. In a similar manner, signals to betransmitted to the network 141 are processed (e.g., modulated andencoded) by the DSP 158 and are then provided to the transmitter 152 fordigital to analog conversion, frequency up conversion, filtering,amplification and transmission to the communication network 141 (ornetworks) via the antenna 156.

In addition to processing communications signals, the DSP 158 providesfor control of the receiver 150 and the transmitter 152. For example,gains applied to communications signals in the receiver 150 andtransmitter 152 may be adaptively controlled through automatic gaincontrol algorithms implemented in the DSP 158.

In a data communications mode, a received signal, such as a text messageor web page download, is processed by the communications subsystem 101and is input to the processing device 180. The received signal is thenfurther processed by the processing device 180 for an output to thedisplay 160, or alternatively to some other auxiliary I/O device 106. Adevice user may also compose data items, such as e-mail messages, usingthe keypad 140 and/or some other auxiliary I/O device 106, such as atouchpad, a rocker switch, a thumb-wheel, or some other type of inputdevice. The composed data items may then be transmitted over thecommunications network 141 via the communications subsystem 101.

In a voice communications mode, overall operation of the device issubstantially similar to the data communications mode, except thatreceived signals are output to a speaker 110, and signals fortransmission are generated by a microphone 112. Alternative voice oraudio I/O subsystems, such as a voice message recording subsystem, mayalso be implemented on the device 20. In addition, the display 160 mayalso be utilized in voice communications mode, for example to displaythe identity of a calling party, the duration of a voice call, or othervoice call related information.

Any short-range communications subsystem enables communication betweenthe mobile device 20 and other proximate systems or devices, which neednot necessarily be similar devices. For example, the short-rangecommunications subsystem may include an infrared device and associatedcircuits and components, or a Bluetooth™ communications module toprovide for communication with similarly-enabled systems and devices.

In accordance with a non-limiting example, FIG. 4 shows an example of acircuit layout on part of the circuit board 67 that can be includedwithin the mobile wireless communications device 20 of FIGS. 1-3, andshowing a front cover removed from a housing to illustrate a surfacemounted microphone 200 and its circuitry and associated noise isolationcomponents as will be explained in greater detail below. The circuitboard 67 includes the radio frequency (RF) circuitry 48, for example,cellular telephone communications circuitry, which is mounted in firstand second isolation shields or “cans” 210,212, as often called by thoseskilled in the art, forming a compartment on the circuit board, eachwhich receive the RF circuitry. Each can 210,212 forms a radio frequencyisolation compartment and may include sides and a top. The first can 210in one non-limiting example includes a transceiver chip set 220, forexample, a transmitter chip, receiver chip, and local oscillator chip asnon-limiting examples with those chips labeled A, B and C. Otherillustrated components could include the various resistors, capacitors,amplifiers, regulators and other circuit components common to thosedevices, but not explained in detail.

Located outside first and second isolation cans 210,212, in thisnon-limiting example but mounted on the circuit board 67, is a liquidcrystal display (LCD) connector 230 and a keyboard connector 232, aswell as associated circuit components 234. These components 230, 232 and234 can be configured in different configurations besides theconfiguration illustrated in the non-limiting example of FIG. 4. Thecompartment within the second isolation can 212 includes a poweramplifier 236 and switch diplexer 238 as an antenna switch through whichRF signals are transmitted and received and connected by a transmissionline with the low pass filter as described in greater detail below.Other components 240 are mounted within the compartment and form theresistors, capacitors, transistors, and inductors necessary to drive theaudio and power circuits for the microphone, power amplifier and otherRF circuits.

In one non-limiting example, a radio frequency isolation shield, formedin the illustrated non-limiting example as a third isolation “can” 250,is positioned at a corner of the second “can” 212, and forms anotherisolation compartment at this corner to provide microphone isolation.The RF shield 212 in one aspect is formed as a separate metallic housingsecured to the circuit board. The smaller RF shield 250 surrounds themicrophone, effectively covering, i.e., shielding the entire microphone.That shield can be similarly formed to other shields. Although a “can”configuration formed as a metallic housing with top and sides is usedfor the RF shields 210, 212, other configurations could be used. Each RFshield as a metallic “can” forms a compartment on the circuit board inone non-limiting example.

As noted before relative to the description of FIG. 4, the poweramplifier 236 and switch 238 are contained within the RF metal shield212 forming a can to isolate the power amplifier and switch.

FIG. 5 is the fragmentary drawing representation looking in a sideelevation view and showing the “can” 212 and a typical LC low passfilter 260 that includes an input 262 and output 264 as input and outputtransmission lines that form the transmission line 266 between the poweramplifier 236 and antenna switch 238 as positioned on the printedcircuit board 67. As shown in FIG. 6, the RF circuitry can includetransmit RF circuitry 48A and receive RF circuitry 48B that connect tothe switch 238. The transmit RF circuitry 48A includes the transmissionline 266 to which the power amplifier 236 connects. The LC low passfilter 260 is connected into the transmission line 266 between the poweramplifier 236 and antenna switch 238 and is formed as first and secondparallel capacitors 270, 272, and a series connected inductor 274in-line with the transmission line 266.

The LC low pass filter 260 shown in FIG. 5 and FIG. 6 is implemented toprovide RF filtering between the RF power amplifier 236 and the antennaswitch 238, that operating typically as a transmit and receive antennaswitch. This filter is typically designed as a 3-pole LC filter as shownin the middle of the transmission line 266. These components are placedin a tight physical space under the RF shield 212 and this transmissionline 266 often can be mismatched at harmonic frequencies and create asignificant standing wave.

It should be understood that the standing wave ratio (SWR) is a ratio ofthe amplitude of a partial standing wave at an antinode (maximum) to theamplitude and adjacent node (minimum) in the transmission line. It isusually a voltage ratio called the voltage standing wave ratio (VSWR).There is typically a forward wave with an amplitude superimposed on areflected wave. These reflections occur because of discontinuities, forexample, imperfections in the transmission line or termination pointswith other than a characteristic impedance. Sometimes two waves caninterfere constructively. The SWR is an indicator of the reflected wavesbouncing back and forth within a transmission line and could correspondto an increase in power in the line beyond the actual transmitted powerand create an increase in RF losses as the increased voltage increasesdielectric losses, and the increased current increases resistive losses.Of course, a matched impedance gives an ideal power transfer whilemismatched impedances give high SWR and increased losses in thetransmission line. Usually, some reflected power in the transmissionline is reflected back at the transmitter and radiated by the antenna.Typically, a Smith chart showing a graph can express the reflectioncoefficient such that a magnitude of zero is a perfect match, and avalue of one is a perfect reflection in some non-limiting examples. Ofcourse, the reflection coefficient can distinguish between short andopen circuits where a short circuit has a value of −1 while an opencircuit has an angle of zero degrees. The return loss of a load with amagnitude of the reflection coefficient is typically expressed indecibels. Return loss is typically a positive number.

As a result, because of the standing waves, the RF shield 212 couplesthe RF harmonics from the power amplifier 236 output directly to theswitch input 238 and bypasses the low pass filter 260. This RF couplingis created by the high RF voltage standing wave ratio (VSWR) such asshown in the Smith chart of FIG. 7. The graph shows the correspondingreturn loss (S11) plot of the LC low pass harmonic filter 260 overfundamental and harmonic frequencies. FIG. 8 is a graph showing thecorresponding insertion loss (S21) plot of the same LC low pass harmonicfilter 260 over fundamental and harmonic frequencies. FIG. 6 shows theinput standing waves and output standing waves as part of the harmonicsignal and the fundamental signal.

In accordance with the non-limiting example, FIG. 9 shows a low passfilter that solves the technical problem addressed above by using an RLClossy low pass filter 280 that significantly reduces the voltagestanding wave ratio (VSWR) at harmonic frequencies, such as shown in theSmith chart of FIG. 10. The graph displays the corresponding return loss(S11) plot of the RLC low pass harmonic filter 280 over fundamental andharmonic frequencies. As illustrated, the RLC low pass filter 280includes a first capacitor 282 series connected to a ground connectedfirst resistor 284, which are in parallel with a second ground connectedcapacitor 286. The inductor 288 is serially connected within thetransmission line 266. A second resistor 290 is connected in parallel tothe inductor 288. These various components are selected to haveresistance, capacitance, and inductance values such that at afundamental frequency, the impedance of the capacitance established bythe capacitors 282, 286 is higher than the first resistor 284 and theimpedance of the inductor 288 is lower than the second resistor 290. Athigher harmonic frequencies, the impedance of the capacitor 282 isreduced, and the first resistor 284 becomes a dominant element forreducing any RF coupling of voltage standing waves of upper harmonicfrequencies from the power amplifier 226 into the antenna switch 238between the input and output of the low pass filter 280 through the RFmetal shield 212 while maintaining transmission of RF signals throughthe transmission line 266 at a desired fundamental frequency. R1 and R2284, 290 have a minimum effect to the performance of the low passfilter, i.e., the insertion loss. At the higher harmonic frequencies, asnoted before, the impedance of the first capacitor 282 is reducedsignificantly, and the first resistor 284 becomes the dominatingelement. The same is true relative to the second resistor 290, which isrelatively lower in impedance than that of the inductor 288. Thus, theRLC low pass filter 280 as described becomes a more dissipative filterat its input, which results in a very low voltage standing wave ratio(VSWR) as shown in FIG. 10. The corresponding insertion loss (S21) plotof the RLC low pass harmonic filter 280 over fundamental and harmonicfrequencies is shown in the graph of FIG. 11.

As a result, the RF shield no longer couples RF harmonics when the poweramplifier output is directly output to the switch input.

Many modifications and other embodiments will come to the mind of oneskilled in the art having the benefit of the teachings presented in theforegoing descriptions and the associated drawings. Therefore, it isunderstood that various modifications and embodiments are intended to beincluded within the scope of the appended claims.

That which is claimed is:
 1. A mobile wireless communications device,comprising: an antenna switch; a power amplifier; a transmission pathcoupling said power amplifier and said antenna switch; and an RF shieldsurrounding said power amplifier, said antenna switch, and saidtransmission path; said transmission path comprising at least oneresistor, at least one inductor and at least one capacitor coupledtogether to define a low pass filter that is configured to reduce RFcoupling of voltage standing waves through the RF shield from the poweramplifier into the antenna switch.
 2. The mobile wireless communicationsdevice according to claim 1, wherein said at least one resistor, atleast one inductor and at least one capacitor comprises: a capacitor anda first resistor in series; and an inductor and second resistor inparallel.
 3. The mobile wireless communications device according toclaim 1, wherein said at least one resistor, and at least one capacitorcomprises a resistor and a capacitor in series therewith.
 4. The mobilewireless communications device according to claim 1, wherein said RFshield comprises a metallic housing.
 5. The mobile wirelesscommunications device according to claim 4, wherein metallic housingcomprises a top and sides extending therefrom.
 6. The mobile wirelesscommunications device according to claim 1, further comprising RFcircuitry coupled to said power amplifier and to said antenna switch. 7.The mobile wireless communications device according to claim 6, whereinsaid RF circuitry comprises RF circuitry for Global System, for Mobile(GSM) signals.
 8. The mobile wireless communications device according toclaim 6, wherein said RF circuitry comprises a transceiver chip set. 9.A mobile wireless communications device, comprising: an antenna; anantenna switch coupled to said antenna; a power amplifier; atransmission path coupling said power amplifier and said antenna switch;and an electrically conductive shield surrounding said power amplifier,said antenna switch, and said transmission path; said transmission pathcomprising at least one resistor, at least one inductor and at least onecapacitor coupled together to define a low pass filter that isconfigured to reduce RF coupling of voltage standing waves through theelectrically conductive shield from the power amplifier into the antennaswitch.
 10. The mobile wireless communications device according to claim9, wherein said at least one resistor, at least one inductor and atleast one capacitor comprises: a capacitor and a first resistor inseries; and an inductor and second resistor in parallel.
 11. The mobilewireless communications device according to claim 9, wherein said atleast one resistor and at least one capacitor comprises a resistor and acapacitor in series therewith.
 12. The mobile wireless communicationsdevice according to claim 9, wherein said electrically conductive shieldcomprises a metallic housing.
 13. The mobile wireless communicationsdevice according to claim 12, wherein metallic housing comprises a topand sides extending therefrom.
 14. The mobile wireless communicationsdevice according to claim 9, further comprising RF circuitry coupled tosaid power amplifier and to said antenna switch.
 15. The mobile wirelesscommunications device according to claim 14, wherein said RF circuitrycomprises RF circuitry for Global System, for Mobile (GSM) signals. 16.The mobile wireless communications device according to claim 14, whereinsaid RF circuitry comprises a transceiver chip set.
 17. A method formaking a mobile wireless communications device comprising: coupling anantenna switch to a power amplifier via a transmission path, thetransmission path comprising at least one resistor, at least oneinductor and at least one capacitor coupled together to define a lowpass filter; and positioning an RF shield surrounding the poweramplifier, the antenna switch, and the transmission path, wherein thelow pass filter is configured to reduce RF coupling of voltage standingwaves through the RF shield from the power amplifier into the antennaswitch.
 18. The method according to claim 17, wherein the at least oneresistor, at least one inductor and at least one capacitor comprises: acapacitor and a first resistor in series; and an inductor and secondresistor in parallel.
 19. The method according to claim 17, wherein theat least one resistor and at least one capacitor comprises a resistorand a capacitor in series therewith.
 20. The method according to claim17, wherein the RF shield comprises a metallic housing.
 21. The methodaccording to claim 20, wherein metallic housing comprises a top andsides extending therefrom.
 22. The method according to claim 17, furthercomprising coupling RF circuitry to the power amplifier and to theantenna switch.
 23. The method according to claim 22, wherein the RFcircuitry comprises RF circuitry for Global System, for Mobile (GSM)signals.
 24. The method according to claim 22, wherein the RF circuitrycomprises a transceiver chip set.